PROJECT SUMMARY CD8 T cells play a critical role in the elimination of infected and cancerous host cells. The success of these immune responses depends on the proper maintenance (survival and proliferation) and precise differentiation of CD8 T cells. Upon antigen stimulation, activated antigen-specific CD8 T cells undergo clonal expansion and differentiate into effector lymphocytes with cytolytic function. Although the majority of effector cells die after antigen clearance, a few antigen-specific cells survive and form long-lived protective memory CD8 T cells. However, during chronic infection and cancer, the immune response is often compromised: CD8 T cells exhibit defects in survival and proliferation, fail to form memory cells and instead are diverted to differentiate into exhausted cells which are characterized by the loss of effector function. Thus, it is of utmost importance to understand the underlying mechanisms that regulate the maintenance and differentiation of CD8 T cells. We have already demonstrated that let-7-mediated posttranscriptional mechanism controls the differentiation and function of effector T cells. Furthermore, our preliminary results suggest that modulation of let-7 levels have a profound impact on the maintenance of T cells, memory formation and diversion into exhaustion. To investigate the molecular mechanisms of let-7 regulation in CD8 T cells we propose the following aims: Aim-1 will determine the molecular basis of let-7-mediated effects on CD8 T cell maintenance: bcl-2-dependent survival and cdc34-driven proliferation. Aim-2 will dissect let-7-mediated mechanisms that guide CD8 T cell differentiation: Eomes-dependent and Eomes-independent molecular programs. To address these aims we will analyze the maintenance and differentiation of CD8 T cells with different levels of let-7 microRNAs into effector, memory and exhausted T lymphocytes using infection and tumor models. We expect to identify novel posttranscriptional mechanisms that regulate the outcome of CD8 T cell-mediated immune responses. It is anticipated that these experiments will define let-7 as a new therapeutic target.